The present invention relates to an amorphous silicon thin film transistor array utilized in an active matrix type liquid crystal display device or the like, and a method for producing the same.
In recent years, a liquid crystal matrix display device, especially a liquid crystal display device of the so-called active matrix type provided with a switching element per each pixel has been under research and development in various places for a thin type picture image display device. An MIS type thin film transistor (hereinafter referred to as TFT) using amorphous silicon (hereinafter referred to as a-Si) has been primarily utilized for the above switching element.
FIG. 8 shows an example of the construction of an active matrix type liquid crystal display device using TFT. In this Figure, when Xi is selected, for example, in the gate wiring 8, the gates of respective TFT 21-a connected thereto are turned on all at once, and through the sources of these TFTs turned on, the signal voltage corresponding to the picture information is transmitted from respective signal lines 12 to the drains of the respective TFT 21-a. A pixel electrode (not shown in the Figure) is connected to the drain, and due to the voltage difference between this pixel electrode and the counter electrode 23 formed on the substrate, which is on the other side, putting the liquid crystal layer 22 therebetween, the optical transparency of the liquid crystal layer 22 is changed to effect picture image display.
When Xi is in a non-selective state, the gate of each TFT 21-a connected thereto is off, and successively, Xi+1 is selected, and the gate of each TFT 21-b connected thereto is turned on, and a process similar to that as described above is effected. Since the voltage difference between both the pixel electrode and the counter electrode 23 is preserved by the liquid crystal layer 22 until the same gate wiring 8 is selected in the next time, the liquid crystal corresponding to each pixel is statically driven, and a display of high contrast can be obtained even after the gate has been turned off.
FIG. 9 is a diagram showing an example of the a-Si TFT used in the above-described TFT 21-a and 21-b. Numeral 1 denotes an insulating substrate such as glass or the like, numeral 2 a gate insulating layer, numeral 3 an amorphous silicon layer, numeral 4 a protective insulating layer, numeral 9 a gate electrode, numeral 12 a source wiring, numeral 13 a pixel electrode, numeral 15 a source electrode, and numeral 16 a drain electrode. As shown in this Figure, a gate electrode 9, a source electrode 15 and a drain electrode 16 are formed putting the gate insulating layer 2 and the amorphous silicon layer 3 therebetween, and the gate electrode 9 is formed on the side nearer to the substrate than the source electrode 15 and drain electrode 16. A TFT having the structure explained above is called a reverse staggered type TFT.
In an active matrix type liquid crystal display device, since the terminal part of the gate wiring should effect the connection to the external circuit at the peripheral part of the substrate, the gate insulating layer 2 or the like must not be formed on the gate wiring terminal part in the peripheral part of the substrate. In usual cases, as shown in FIG. 10, when a gate insulating layer or the like is formed, a metal.. mask 14 is used to mask the peripheral part of the insulating substrate 1 such that a gate insulating layer or the like is not formed at this masked part.